SHIBAM, Yemen -- Yemeni authorities have determined an explosion in Shibam that killed four South Korean tourists was a suicide bombing by Islamic terrorists, sources say.

An unnamed source in Yemen's Ministry of Foreign Affairs and Trade said a preliminary investigation indicated the Sunday blast was set off deliberately, while local reports said police had arrested 12 militants with alleged links to terrorist groups such as al-Qaida, The Korean Times reported.

"The preliminary investigation by the Yemeni Interior Ministry found that it was a deliberate terrorist attack," the unnamed official told Yonhap.

Tourists who survived a suspected a suicide bombing returned home Tuesday, with two of the wounded victims taken to a hospital by ambulance shortly after arriving at an airport west of Seoul, the South Korean news agency Yonhap said.